This application claims the priority of British patent document 0014064.0, filed Jun. 10, 2000 (PCT International Application No. PCT/GB01/01769), the disclosure of which is expressly incorporated by reference herein.
This invention relates to ring-wing aircraft and is suited particularly, although not exclusively, to use in micro-unmanned air vehicles (UAV""s) incorporating ring-wings. However, it will be appreciated that the invention is equally well suited to use in ring-wing aircraft of any size, whether they be manned or unmanned.
Unmanned air vehicles have found a number of applications, where they have been used to carry a wide variety of payloads. In the absence of a pilot, the aircraft may be scaled down, typically to a size of 150 mm or less. The reduction in size and the absence of extra load in the form of a pilot fit in well with the constant drive to produce lighter aircraft because of their inherent efficiency advantages.
In terms of flexibility it is advantageous for UAV""s to be able both to hover and to fly horizontally at high speed. This criterion has led to the development of ring-wing aircraft, i.e. an aircraft having a propeller or other propulsion system mounted within a duct defined by a ring-shaped wing of substantially circular cross-section. A ring-wing aircraft is disclosed in U.S. Pat. No. 5,295,643 to Hughes Missile Systems Company, although the scale of the aircraft is not disclosed. The Hughes aircraft has a xe2x80x98toroidalxe2x80x99 ring-wing that defines a duct (although the ring-wing is generally flat in cross-section so that it can be considered as essentially an annular cylinder). A propeller, stators and vanes are located within the duct. In the Hughes aircraft, the stators serve to straighten the swirling air produced by the rotating propeller and hence help to balance the torque applied to the aircraft by the rotating propeller. The vanes are used as control surfaces for controlling the pitch and yaw of the aircraft when airborne.
The use of a ring-wing has benefits that include reducing the noise signature of the propeller and providing a protective housing for the propeller that guards against both damage to the propeller and damage to anything or anyone that may otherwise come into contact with the propeller. However, the main advantage of the ring-wing design is that it can be used both for hovering, by orienting the propeller horizontally, and for horizontal flight by orienting the propeller vertically. In the Hughes aircraft, the transition from vertical to horizontal flight is effected by use of the vanes.
However, the Hughes Patent does not discuss any aspects of roll pertinent to their aircraft, nor does it contain any detailed discussion on any aspects of yaw or pitch. Clearly these are important aspects on the design of a ring-wing UAV as these aircraft must be controlled either autonomously or by remote control, and so control and stability of the aircraft must be enhanced, say, over a manned vehicle where the pilot will have direct feedback from his sense of balance as to the attitude of the aircraft. Where autopilots are used, natural stability of the UAV is highly important as it reduces the complexity, size and, critically, the mass of the autopilot system to be installed on the aircraft.
Natural stability in the pitch and yaw planes is achieved by placing the centre of mass of the aircraft ahead of the aerodynamic neutral points in the pitch and yaw planes. The shape of the ring-wing means that the aerodynamic pressures acting on all portions of the ring-wing have a line of action that inherently passes through the centre of the ring. The symmetry of the continuous ring-wing ensures that these pressures give rise to no contribution to rolling moment. It will be appreciated that the ring-wing need not be continuous: the ring may be divided into two or more sections and there will still be no rolling moment contribution from the aerodynamic pressures.
However, the inherent neutral symmetry of ring-wing aircraft can be considered problematic. This is because any disturbance about the roll axis leaves the aircraft in an attitude without any incremental aerodynamic rolling moment to restore it to its undisturbed conditionxe2x80x94there is no tendency for the aircraft to right any induced roll. This tendency freely to roll can lead to a requirement for a flight control system to provide a means of controlling the vehicle, thereby adding complexity, and quite often mass, to the aircraft.
An object of the invention is to overcome the disadvantages of previous designs of ring-wing UAV""s described hereinabove.
From a first aspect, the present invention resides in an aircraft comprising a ring-wing defining a duct with a longitudinally extending central axis, propulsion means located within the duct and moveable aerofoils for controlling the aircraft in flight, the ring-wing being truncated obliquely at one end, that end being the rear when in horizontal flight, to form a ring-wing with opposed sides of unequal length.
It should be noted that the term ring-wing is intended to encompass rings of substantially circular cross-section, both continuous and interrupted, i.e. where segments of the ring have been omitted. This is best done to leave a wing symmetrical about its central axis.
It will be appreciated that by truncating the ring-wing obliquely at its rear produces and aircraft with a centre of mass offset from the central axis of the ring-wing. The pendulum effect will ensure that the centre of mass will always be at the lowest height possible when the aircraft is airborne. Hence, should the aircraft roll, the rolling moment due to the increased height of the centre of mass will produce a rolling motion of the aircraft so that the centre of mass returns to the lowest height possible. It will be appreciated that the further the centre of mass is offset from the central axis of the ring-wing, the greater the tendency for the aircraft to roll to correct any disturbance about its roll axis. Therefore the aircraft has a preferred orientation, and the control surfaces can be oriented with respect to this preferred orientation. Roll stability is thus achieved through the restoring moments provided by the offset centre of gravity rather than through aerodynamic restoring moments.
In addition, truncating the ring-wing at its rear rather that its front is advantageous as it produces a centre of mass ahead of the aerodynamic neutral points in the pitch and yaw planes when the aircraft is engaged in high-speed forward flight. It should be appreciated that the applicant has realised that a simple modification of the basic ring-wing shape is highly beneficial because in addition to offsetting the centre of mass from the central axis of the ring-wing to provide self-righting roll control, it also moves the centre of mass towards the front of the aircraft, i.e. the simple modification provides much improved stability in all three axes.
Optionally, the ring-wing is truncated by a planar slice through the ring-wing. It is preferred that the planar slice makes an angle of between 25xc2x0 and 65xc2x0 to the longitudinally extending central axis. It is further preferred for the angle to be between 25xc2x0 and 45xc2x0 and yet further preferred for the angle to be substantially equal to 45xc2x0. Optionally, the ring-wing is truncated by a curved slice, the angle made with the ring-wing being relatively shallow at the longer side of the ring-wing and relatively steep at the shorter side of the ring-wing. This arrangement conveniently produces a greater shift in the centre of mass to the shallow end.
The payload and other components of the aircraft that do not need to be located on the central axis of the ring-wing (e.g. remove control receiver, batteries or fuel tank, motor speed controller, etc.) constitute the substantial majority of the overall mass of the aircraft and, thus, should be located offset from the central axis of the ring-wing. In order to maximise the pendulum effect, it is advantageous to house components and/or any payload in a compartment that is external to the longer side of the ring-wing. Alternatively, or in addition, components and/or any payload may be housed in a compartment provided within the longer side of the ring-wing. Advantageously, the compartment may be provided at one end of the ring-wing, that end being the front when in horizontal flight, as this configuration assists stability in all three axes.
The aircraft may have a pitch-controlling aerofoil component provided offset from the central axis of the ring-wing. Optionally, it may be positioned to reside substantially flat with the longer side of the ring-wing when not deployed. Advantageously, it may be provided as an element of the longer side of the ring-wing.
Conveniently, the aircraft may further comprise two orthogonal vanes wherein one vane provides aerodynamic lift in the same direction as the pitch-controlling aerofoil. These vanes may primarily control the roll and yaw attitude of the aircraft, and they may operate in conjunction with the pitch-controlling aerofoil in certain manoeuvres, to effect a turn for example.
Optionally, any aircraft defined hereinabove is unmanned. The diameter of the ring-wing may be less than 50 cm, although a diameter of less than 25 cm is currently preferred.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.